1. Field of the Invention
This invention relates to a method of metering a coating solution in which, after a coating solution is applied to a continuously running web, surplus coating solution on the web is scraped off with a coil bar to form a coated film or layer to a desired thickness, and to an apparatus for practicing the method.
2. Description of the Prior Art
The term "web" as herein use is not limited and is intended to mean a relatively long, flexible, belt-shaped support such as: a plastic film of cellulose triacetate, polyamide, polyimide, polycarbonate, polyethylene terephthalate or polyvinyl chloride; paper; synthetic paper; a metal foil of aluminum or copper; sheet of glass or ceramic and the like.
The term "coating solution" as used herein is not limited and is intended to mean a variety of coating solutions different in composition depending on the purpose of use. Examples of the "coating solutions" are coating solution for forming a photosensitive emulsion layer, a base layer, a protective layer and a back layer as are used in a conventional photographic photosensitive material; coating solutions for forming a magnetic layer, a base layer, a wetting layer, a protective layer and a back layer as used in a conventional magnetic recording medium; and coating solutions for forming an adhesive layer, a coloring layer, a rust-proofings layer, etc. These coating solutions are made up of essential components, a binder, and, if necessary, water or organic solvent or dispersion containing various additives.
In order to form a coated film on the above-described web to a desired thickness, the following methods are now extensively employed: Conventional coating means such as a dip, reverse roll, gravure roll, extrusion hopper or slide hopper are used to apply an excess of coating solution to the web which is run continuously. Then, metering means such as an air knife, blade or coil bar (or wire bar) are contacted with the coated layer to scrape off surplus coating solution to thereby coat the web with the coating solution to the degree or thickness.
In coating a web with a coating solution of high viscosity, such as a magnetic solution for forming a magnetic recording medium, a coil bar has been extensively employed as the metering means because it is simple in structure and can be readily handled as compared with the air knife or blade, and the coated surface is relatively stable in quality.
A conventional coating method using the coil bar is as shown in FIG. 1.
As shown in FIG. 1, a web 1 is run continuously in the direction of the arrow, and a coating solution 3 is applied to the lower surface of the web 1 to a thickness larger than finally required to form a coated film 5 in the liquid state. Surplus coating solution 7 is scraped off by the coil bar 6 so that a coated film 8 is formed whose thickness is regulated by the gap defined by the cylindrical wall of the coil bar 6 and the surface of the web 1. If desired or necessary, for instance in the case of forming a magnetic layer, the product is subjected to magnetic field orientation and dried and is then wound. In FIG. 1, reference numeral 4 designates guide rolls rotating in the direction of the arrow.
The coil bar 6, as shown in FIG. 2, is made up of a core material or a rod member 9 and a wire 10 which is wound in the form of a single coil on the cylindrical wall of the rod member 9 with the turns being in close contact with one another. In general, rod member 9 is a stainless steel, iron or brass rod 1 to 3 mm in diameter, and wire 10 is a stainless steel, trifluoroethylene homopolymer, tetrafluoroethylene homopolymer, or tetrafluoroethylene-hexafluoropropylene copolymer wire 0.04 to 0.05 mm in diameter. In addition to the above-described coil bar, a small coil bar less than 4 mm in maximum coil radius which was proposed by the present inventors may be employed (see Japanese patent application No. 41060/1981).
In general, the web is dried after being subjected to metering as described above, and is then wound. For instance in the case of manufacturing a magnetic recording medium, the web is subjected to special processes such as a magnetic field orientation process and a surface smoothing process.
In order that a coated film of excellent surface quality is obtained by effectively carrying out the metering operation with a coil bar, the coil bar should be wet with the coating solution during the metering operation. Usually the coil bar is wetted by the coating solution during the metering operation. However, when a coating solution of high viscosity is used or coating speed is increased, the coil bar is not sufficiently wet by the coating solution. Accordingly, in this case, rib-shaped stripes are formed on the coated surface after the metering operation, or the coated surface is not smooth. Furthermore, the coating solution on the coil bar hardens and sticks to the coated surface which has been metered, thus degrading the coated surface.
In order to eliminate the above-described difficulties, a method as shown in FIG. 3 has been proposed. In this method, a coil bar 12 as described above is rotatably supported by a holder 13. The holder 13 has a solution supplying slot 14 which opens below the coil bar 12. A solution, which is substantially the same in composition as the coating solution, is supplied through the slot 14 to the coil bar 12 to wet the same during metering. In FIG. 3, reference numeral 1 designates a web which is run continuously in the direction of the arrow. The solution which has wet the coil bar drops down the rear wall of the holder (as indicated at 16) or drops down the front wall of the holder together with coating solution which has been subjected to metering (as indicated at 15) to be recovered or recycled.
It is true that this method is somewhat effective, but, it still suffers from the problem that the solutions flow down the front and rear walls of the holder irregularly, i.e., the solutions do not flow down the walls uniformly, so that the solutions may dry. Thus, even this coil bar wetting method still suffers from problems.